LIGHT AND COLOUK. 315 
sider green as a primary colour, consequently making their 
actual number four. 
This is not the place, however, for discussing this delicate 
question ; we have chiefly to prove that colour has no existence 
without Light. Another evidence of this is given in the colours 
of thin films. A soap bubble blown by a child, and floating in 
the sun-beam, becomes a study to the philosopher. If we mix 
soap with a solution of sugar in water, we may give consider- 
able permanence to the filmy globe, and study all its beautiful 
changes, as the thickness of the film varies. A drop of colour- 
less oil let fall upon water spreads out in a thin veil, and sends 
to the eye a glorious series of colours. Even films of air do 
the same thing. If a plano-convex lens is placed on a convex 
lens it is obvious that they can only touch at one point, and 
that films of air varying in thickness surround that point. The 
result of this is a system of circular coloured rings. They are 
of one set of colours udien seen by reflection, and of another set 
when viewed by transmission. Looking - at those rings, they 
will be in one condition, as shown in Fig. 3. Looking through 
them, they will appear as in Fig. 4, or complementary to the 
first. These remarkable colours vary with every variation in 
the thickness of the film of air. 
Another illustration of the dependence of colour on Light is 
produced by passing the ray of light admitted through our 
pin-hole in the dark room, near the edge of any body. This 
effect is termed the inflexion of Light, or diffraction. If a 
short-focus lens be placed in the hole through which the Light 
is admitted, the effect is more intense. If a needle be held, 
in this beam, its shadow will be found to be surrounded with 
three fringes : the first giving all the colours of the spectrum ; 
the second, blue, yellow, and red; and the third fringe, pale 
blue, pale yellow, and pale red. It is impossible to represent 
these fringes by artificial colouring ; but the beautiful curves 
formed are shown in Fig. 5. If the ray of Light be received 
on a glass rod blackened, and Ave look at it through a fine slit 
in a card, the appearance will be, as shown in Fig. 6 — white 
light occupying the centre of the whole, and spectra separated 
by black lines extending on either side. 
The colours of fibres and of grooved surfaces are easily seen. 
If Ave dust some hair-poAvder or lycopodium upon a glass plate, 
and look through it at a candle, the flame will appear surrounded 
by a halo. Fibres of silk or avooJ, spread out closely together, 
give rise to similar coloured rings. The colours of mother-of- 
pearl are due to the infinite number of fine grooves upon its 
surface. If Ave melt a little Avliite wax, and pour this on the 
mother-of-pearl, it receives impressions of those fine grooves, 
and, if removed, Avhen cold it Avill be as iridescent as the shell 
